1. Field of the Invention
This invention relates generally to read sensors having an MTJ (magnetic tunneling junction) structure and particularly to the formation and composition of the free layer and capping layers of such sensors.
2. Description of the Related Art
In simplest form, the magnetic tunneling junction, magnetic random access memory cell (MTJ MRAM cell) is formed by patterning a stack of horizontal layers comprising two plane, parallel magnetic layers, separated by an insulating spacer layer of such thinness as to permit the quantum mechanical tunneling of electrons through it. Each magnetic layer in the stack is given a unidirectional magnetic moment within its plane of formation and the relative orientations of the two planar magnetic moments determines an electrical resistance that is experienced by a current that passes from magnetic layer to magnetic layer through the spacer layer (hereinafter referred to as the tunneling barrier layer). The physical basis for the variable resistance (the MTJ effect) is the fact that the current of conduction electrons is spin polarized by its interaction with the magnetic moment of the first of the magnetized layers it passes through. In order for this polarized current to tunnel through the tunneling barrier layer and enter the second of the magnetized layers, the second magnetized layer must have available states in its conduction band to accept the electrons. The availability of such states, in turn, depends on the magnetization direction of the second magnetized layer. The probability of an electron successfully tunneling through the tunneling barrier layer depends on the availability of those states and, therefore, depends on the relative magnetization directions of the layered configuration. In effect, the configuration is a variable resistor that is controlled by the angle between the magnetizations.
In what is called a spin-filter configuration, one of the two magnetic layers in the MTJ cell has its magnetization fixed in spatial direction (the pinned layer), while the other layer (the free layer) has its magnetization free to move in response to an external magnetic stimulus. If the magnetization of the free layer is only permitted to take on two orientations, parallel or antiparallel to that of the pinned layer, then the MTJ cell will have only two resistance values: a high resistance, Rap, when the magnetizations are antiparallel and a low resistance, Rp, when they are parallel. Thus, these two values can be associated with a logical 1 (high resistance) and a logical 0 (low resistance) and the cell can be used to store information as part of an MRAM device (the device denoting the cell, or array of cells, plus associated circuitry).
The efficacy of the MTJ cell as an information storage device depends on several factors. First, once the cell has been placed in its high or low resistance state, it should be stable until it is placed in an alternative state. Second, since the cell is ultimately part of an array of many cells, their high and low resistance values should be closely comparable to within a fairly rigid set of error bars. In fact, the way a cell is determined to be in its high or low resistance state, is to compare it to a standard cell in the array. If the covariance of the statistical variations of parallel resistances, Rp and antiparallel resistances, Rap among cells, denoted respectively Rp—cov and Rap—cov, is too great, then the reading of stored logical values will be in error.
The high speed version of MTJ MRAM devices is based on the integration of MTJ devices with silicon based CMOS circuitry. In particular, each MTJ cell is associated with a single CMOS access transistor (1T1MTJ architecture) so that a current can be passed through the cell and the resistance value of the cell can be read. For writing information onto a cell, the magnetization of its free layer must be changed from parallel to antiparallel or vice versa. This change in magnetization direction is accomplished by applying currents to an orthogonal matrix of current carrying lines, wherein one cell is positioned adjacent to a crossing of a pair of lines (a cross point). One of the pair of lines, called the bit line, provides an induced magnetic field that is parallel to the easy axis of the free layer (the axis parallel to the magnetization direction). The other of the pair of lines, called the digit line, provides an induced magnetic field that is along the hard axis direction of the free layer, which is perpendicular to the easy axis direction. When both lines are producing their fields, the resulting field is strong enough to switch the magnetization direction of the free layer.
A figure of merit for an MTJ cell is the TMR ratio, denoted as dR/R, which is actually (Rap−Rp)/Rp. In order for an array to have a good read operation margin (a margin of error that insures that comparisons between the individual cells and the standard cell will yield correct assessment of high and low resistance values), the ratio of TMR/Rp—cov (or TMR/Rap—cov) should be no less than 12, preferably >15 and even more preferably >20. For a good write operation, a high AQF (array quality factor), defined as Hc/σHc, having a value ≧20 is desirable. Here Hc is the mean value of the switching magnetic field and σHc is the standard deviation of the switching fields across the array. In this regard see D. W. Abraham (U.S. Pat. No. 7,026,673).
Early versions of a 4 Mb MTJ MRAM chip comprised MTJ stacks patterned in an approximate C shape of dimension 0.3×0.6 microns. The stack, denoted as configuration (1) below, illustrated in FIG. 1 and further described in Horng et al. (US Published Pat. Appl. 2008/0088986) which is fully incorporated herein by reference was:BE/NiCr45/MnPt150/CoFe21/Ru7.5/CoFeB15-CoFe6/Al8-ROX/Ni(88%)Fe(12%)28-NiFeHf35(free layer)/Ta  (1)Where BE (1) is a bottom electrode, NiCr (2) is a seed layer of 45 angstroms thickness, MnPt (3) is an antiferromagnetic pinning layer of 150 angstroms thickness, CoFe (4) is an outer pinned layer of 21 angstroms thickness, Ru (5) is a coupling layer of 7.5 angstroms thickness, CoFeB—CoFe is an inner pinned bilayer (6a), (6b) where the CoFeB is of 15 angstroms thickness and the CoFe is of 6 angstroms thickness so that the (4), (5), (6a)(6b) structure is a synthetic antiferromagnetically pinned layer (SyAP), Al ROX (7) is 8 angstroms of aluminum, oxidized by radical oxidation (ROX) to create an AlOx tunneling barrier layer, Ni(88%)Fe(12%)-NiFeHf (8a) (8b) is a free layer bi-layer in which a first NiFe layer (8a) having Ni with 88% atom percentage and Fe with 12% atom percentage is 28 angstroms in thickness and a second, (Hf doped NiFe) NiFeHf layer is 35 angstroms in thickness and Ta (9) is a capping layer. It is to be noted that the free layer is composed of material of low intrinsic magnetization so that it must be made thick. This added thickness reduces the statistical variation in magnetic switching thresholds, σHc, thereby producing a high value of AQF>20. This particular technology is implemented in the (1T2MTJ) configuration where two MTJ cells are associated with a single access transistor and the free layers of the two MTJ elements are oppositely aligned. For reference purposes, the product of cross-sectional area and resistance, RA is approximately 1000 ohm-μm2 and intrinsic TMR is about 45%. The low magnetization free layer allows a very low magnetostriction to be obtained (X, approx. 2-5×10−7). For this product, mean TMR at device read operation (with 300 mV bias) is approximately 25% and mean Rp—cov is approx. 1.5%, yielding a read margin of TMR/Rp—cov of approx. 16. For the oval shaped cell Rp—cov is about 1.2%. so the read margin is greater than 20.
Using smaller design/process technologies, a 16 Mb MTJ MRAM is now being fabricated in which the cell size is on the order of 0.2×0.4 μm and is C-shaped in cross-section. Initially, the stack configuration of (1) was used. With that configuration, mean Rp—cov is increased to 1.7% and mean TMR at 300 mV bias is decreased slightly to 22-23%. Therefore, the read margin becomes approximately 13, which is too marginal for production purposes. For production quality 16 Mb MTJ MRAM, it is necessary to improve TMR in order to improve the device read margin. This will require an MTJ cell with an intrinsic TMR≧50%.